1. Field of the Invention
The present disclosure is generally related to digital communications and, more particularly, is related to systems and methods for encoding digital communications.
2. Related Art
Communication networks come in a variety of forms. Notable networks include wireline and wireless. Wireline networks include local area networks (LANs), digital subscriber line (DSL) networks, and cable networks, among others. Wireless networks include cellular telephone networks, classic land mobile radio networks and satellite transmission networks, among others. These wireless networks are typically characterized as wide area networks. More recently, wireless local area networks and wireless home networks have been proposed, and standards, such as Bluetooth and IEEE 802.11, have been introduced to govern the development of wireless equipment for such localized networks.
A wireless local area network (LAN) typically uses infrared (IR) or radio frequency (RF) communications channels to communicate between portable or mobile computer terminals and stationary access points or base stations. These access points are, in turn, connected by a wired or wireless communications channel to a network infrastructure which connects groups of access points together to form the LAN, including, optionally, one or more host computer systems.
Wireless protocols such as Bluetooth and IEEE 802.11 support the logical interconnections of such portable roaming terminals having a variety of types of communication capabilities to host computers. The logical interconnections are based upon an infrastructure in which at least some of the terminals are capable of communicating with at least two of the access points when located within a predetermined range, each terminal being normally associated, and in communication, with a single one of the access points. Based on the overall spatial layout, response time, and loading requirements of the network, different networking schemes and communication protocols have been designed so as to most efficiently regulate the communications.
IEEE Standard 802.11 (“802.11”) is set out in “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications” and is available from the IEEE Standards Department, Piscataway, N.J. 802.11 permits either IR or RF communications at 1 Mbps, 2 Mbps and higher data rates, a medium access technique similar to carrier sense multiple access/collision avoidance (CSMA/CA), a power-save mode for battery-operated mobile stations, seamless roaming in a full cellular network, high throughput operation, diverse antenna systems designed to eliminate “dead spots,” and an easy interface to existing network infrastructures.
The 802.11a standard defines data rates of 6, 12, 18, 24, 36 and 54 Mbps in the 5 GHz band. Demand for higher data rates may result in the need for devices that can communicate with each other at the higher rates, yet co-exist in the same WLAN environment or area without significant interference or interruption from each other, regardless of whether the higher data rate devices can communicate with the 802.11a devices. It may further be desired that high data rate devices be able to communicate with the 802.11a devices, such as at any of the standard 802.11a rates.
A wireless channel may subject the transmitted signals to severe and time-varying attenuation of random nature. For this reason, channel coding, or error correction coding (ECC), which introduces precomputed redundancy on a raw information bit stream, is an integral part of a baseband processor for a wireless modem. A class of channel codes, known as low-density parity-check codes (LDPCCs) achieves this error correction coding in a manner that is close to theoretical limits. An LDPC code includes parameters (n,k) where n is the block length (# bits) and k is the number of information bits encoded per block. Traditional block encoders add a fixed number of parity bits, m=n−k, to each block of k information bits to form an n-bit encoded block with code rate R=k/n.
For a given rate, the error correcting capability of an LDPC code improves with the blocklength, n. LDPC codes are typically decoded via an iterative algorithm, which improves the reliability of bit decisions at each pass. With each iteration, the performance of the decoder improves, with the improvement diminishing as more and more number of iterations are performed. After a number of iterations, the performance of the decoder ceases to improve for all practical purposes, and the decoder is said to have “converged.” The number of iterations required for convergence is a property of the code itself, as well as the specific channel for which it is used. The decoder performance of the LDPC code is thus a function of the number of iterations that can be performed. For a given encoder rate R, the upper limit for a decoding iteration is governed by the number of parity bits, determined by the equation (1−R)×n. Therefore, while it is desirable to use an LDPC code with as large a blocklength as possible, higher blocklengths imply fewer iterations per unit time, which means that the decoder may not harvest the superior error correcting capability of the code. On the other hand, a code with a small blocklength may inherently lack the needed error correcting capability, even if the decoder can implement many fast iterations.
One challenge in a packet-based WLAN radio system, such as, for example, one compliant with 802.11, is to pick an LDPC code block size and a number of iterations to best fit the packet size (total number of available coded bits) while balancing the practical complexity of the decoder. As the transmission data rate increases, the decoder must run faster on average to keep up with data flow. For typical LDPC codes of interest, the decoder may use a large degree of parallelism to perform a desired number of decoding iterations on each received soft codeword. Thus the upper limit of decoding speed is governed approximately by the product of the maximum average coded transmission rate, the number of parity bits per block (1−R)×n, and the number of decoding iterations performed per block. To keep the bit error rate performance (or the code block error rate performance) approximately constant across the packet, the codewords in a packet structure may be of approximately equal size and equal rate. They are decoded using an equal number of decoding iterations. Otherwise the weakest code block in the packet may dominate the overall packet error rate.
Another challenge for the decoder in the WLAN radio is to be able to promptly complete the decoding at the end of reception of a packet so that a return acknowledgement (e.g., ARQ mechanism) can be immediately sent back to the transmitter. Some WLAN radio systems rely on this “ARQ” mechanism to communicate packet errors and instigate retransmission of the packet in the event of an error. The minimum time allowed for this varies according to each transmission standard, but can be as short as 6 μs or so for next generation 802.11 radios. The time between end of reception and the transmission of an acknowledgement is “dead” airtime and thus contributes to network overhead. Therefore, the minimum interframe transmission time (SIFs) for acknowledgement may be optimized in the standard to be as short as possible within practical constraints.
Increasing the data rate and allowing more effective use of bandwidth for devices operating in these bands enables more efficient communications. A higher data rate may enable service providers to more effectively use their allotted spectrum. Consumers may realize a cost savings as well.